I am loading a 48000Hz sweep from 0-24000Hz. I view the Sonogram for the entire duration and find that the upper frequency is 12000Hz and not 24000Hz. The test tone's frequency content for the second half of the sweep (12000 -24000Hz) is folded downwards. This is what the sonogram looks like: and the test asset is here for anybody that wants to try it locally http://www.digenis.co.uk/random/0-24000.wav

Is this a genuine bug or am I missing something? Having compared it with Audition I am leaning towards the former.

click on the small "gearbox" icon and change the maximal frequency to 24kHz, this will give you the expected result.

By default, sounds are subsampled by a factor two before the sonogram is computed (since most sounds of interest have not much interesting frequency content above 10kHz so you get better resolution in the low frequencies), this results in the "folding" you observed, see